Sub Applications - Mounts

Warning

The current page still doesn’t have a translation for this language.

But you can help translating it: Contributing.

If you need to have two independent FastAPI applications, with their own independent OpenAPI and their own docs UIs, you can have a main app and “mount” one (or more) sub-application(s).

Mounting a FastAPI application

“Mounting” means adding a completely “independent” application in a specific path, that then takes care of handling everything under that path, with the path operations declared in that sub-application.

Top-level application

First, create the main, top-level, FastAPI application, and its path operations:

  1. from fastapi import FastAPI
  2. app = FastAPI()
  3. @app.get("/app")
  4. def read_main():
  5. return {"message": "Hello World from main app"}
  6. subapi = FastAPI()
  7. @subapi.get("/sub")
  8. def read_sub():
  9. return {"message": "Hello World from sub API"}
  10. app.mount("/subapi", subapi)

Sub-application

Then, create your sub-application, and its path operations.

This sub-application is just another standard FastAPI application, but this is the one that will be “mounted”:

  1. from fastapi import FastAPI
  2. app = FastAPI()
  3. @app.get("/app")
  4. def read_main():
  5. return {"message": "Hello World from main app"}
  6. subapi = FastAPI()
  7. @subapi.get("/sub")
  8. def read_sub():
  9. return {"message": "Hello World from sub API"}
  10. app.mount("/subapi", subapi)

Mount the sub-application

In your top-level application, app, mount the sub-application, subapi.

In this case, it will be mounted at the path /subapi:

  1. from fastapi import FastAPI
  2. app = FastAPI()
  3. @app.get("/app")
  4. def read_main():
  5. return {"message": "Hello World from main app"}
  6. subapi = FastAPI()
  7. @subapi.get("/sub")
  8. def read_sub():
  9. return {"message": "Hello World from sub API"}
  10. app.mount("/subapi", subapi)

Check the automatic API docs

Now, run uvicorn with the main app, if your file is main.py, it would be:

  1. $ uvicorn main:app --reload
  2. <span style="color: green;">INFO</span>: Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

And open the docs at http://127.0.0.1:8000/docs.

You will see the automatic API docs for the main app, including only its own path operations:

Sub Applications - Mounts - 图1

And then, open the docs for the sub-application, at http://127.0.0.1:8000/subapi/docs.

You will see the automatic API docs for the sub-application, including only its own path operations, all under the correct sub-path prefix /subapi:

Sub Applications - Mounts - 图2

If you try interacting with any of the two user interfaces, they will work correctly, because the browser will be able to talk to each specific app or sub-app.

Technical Details: root_path

When you mount a sub-application as described above, FastAPI will take care of communicating the mount path for the sub-application using a mechanism from the ASGI specification called a root_path.

That way, the sub-application will know to use that path prefix for the docs UI.

And the sub-application could also have its own mounted sub-applications and everything would work correctly, because FastAPI handles all these root_paths automatically.

You will learn more about the root_path and how to use it explicitly in the section about Behind a Proxy.